Method of improving intelligence under random noise interference



Dec. 30,

L. J. FOGEL METHOD OF IMPROVING INTELLIGENCE UNDER RANDOM NOISEINTERFERENCE Filed April 2, 1954 F/G. l

v GATING 1%- AMPLIFIER CIRCUIT Jq PEAK POWER DETECTING CIRCUIT F/6.2- N2I24 n4 Low LEVEL GATING D|2 AM I:(

no SQUELCH CIRCUIT H6 PEAK POWER DETECTING CIRCUIT Low LEvEL GATING D'T' souELcH CIRCUIT q 2Io- 2IG 220 ZIB-I' Low PEAK POWER PASS voETEcTING FILTER CIRCUIT INVENTOR,

LAWRENCE J. FOGEL.

A TTOR/VE).

United States Patent METHOD OF IMPROViN G INTELLIGENCE UNDER RANDOMNOISE INTERFERENCE Lawrence J. Fogel, Jackson Heights, N. Y. ApplicationApril 2, 1954, Serial No. 420,764

' 3 Claims. c1. 179-1 (Granted under Title 35, U. s. Code 1952 sec. 266)The invention described herein may be manufactured and used by or forthe Government for governmental purposes, without the payment of any,royalty thereon.

This invention relates to a method for improving the intelligibility ofspeech and more particularly to a method of improving theintelligibility of speech being amplified by electronic circuitry andhaving low signal to noise ratio.

' The problems of improving the relative quality of the signal inelectronic transmission of sound has been ap proached in many differentWays. There are many systems for reducing noise and some for improvingintelligibility by reducing noise, but most of them appear to lose sightof the basic premise that the improvement of intelligibility in soundtransmission depends to a large extent on the transmission of consonantcharacteristics which provide a high degree of intelligibility as compared with vowel sounds which, contrarily, have the larger amount ofsound energy.

7 The basic noise or scratch reducing systems, standard in-audioamplifiers for many years, use a low-pass filter to reduce the highfrequency noise energy but do this at the expense of the consonantenergy which would improve intelligibility. Other more recent noisesuppressing systems pass all the sound energy during a high level energyinterval then cut off the higher frequencies or the entire signal whenit falls below a certain level. These are also predominately actuated byvowel sounds and lose the value of consonant energy as far as theintelligibility of threshold signals is concerned.

These suppressing systems have also been applied to amplifiers wherespeech is divided into bands. Such systems include: the patent toLlewellyn 1,968,460 where each sub-band of speech energy is separatelysquelched, which favors the vowel sounds; the patent to Beers 1,961,329where the high frequency is decreased when the signal is low, whichagainfavors the vowel'sounds and the patents to Hammond 2,008,825 andDoba 2,173,472 which divide the speech energy into bands with expansionand compression within each band to accentuate the stronger parts. Allof these systems appear to be actuated by the vowel sounds and toaccentuate them at the expense of the consonants which are moreimportant as far as intelligibilityis concerned.

The method of improving intelligibility relied on here ,is based on thesuppression of the peaks of energy ,which are dominant vowel sounds sothat the remainder 'ofthe signal and even some of the noise, whereimportant consonant sounds may be partially masked, may be accentuated.This method relies on the psycho-acoustical ability of the mind to fillin short gaps in sound that are omitted from a familiar pattern ofspeech.

It is therefore an object of this invention to provide a means forimproving the intelligibility of speech.

It is a further object of this invention toprovide a means for theimprovement of intelligibility of speech under high ambient noiseconditions. I I

It is afurther object of this invention to j provide a means forimproving the intelligibility of speech by reice . soundswherein'maximum intelligibility lies.

It is a further object of this invention to provide means for improvingtheintelligibility of sounds by constructing an amplifier to reducepeaks of; energyWhiGh contain predominate vowel energy so that theremaining consonant energy may be accentuated. I Y

It is a further object of this invention to provide a systemfor-supplementing a standard noise suppressor by eliminating the peaksof energy as well as the nulls of energy. K

Other and further objects of this invention will become apparent fromthe following specification and the drawing in which Fig. 1 represents ablock diagram of a typical circuit, Fig. 2 represents an additionalcircuit and Fig.- 3 represents a circuit'for use with a standard'squelching system I Referring more particularly to Fig.1, a system isshown including a microphone IOen erg iZin g'an amplifier 12 whosesignal goes through a gating circuit 14 to a loud- I speaker or otherelectromechanical transducer 16: The signal in the amplifier 12actuate's a peak detecting circuit 18 which supplies a gating signal tothe gating amplifier 14. In operation, a signal is applied by themicrophone to the amplifier 12 which may include vacuum 'tubes or otherdevices well known in the art. The gating circuit 14 may alsoincludewell known electronic circuitry. The

peak detecting circuit 18"monitors the signal infthe am plifier 12 andturns on and oil the gating'circuit to pass the lower component of thesignal rather thanthe higher component to the loud speaker 16. I

It should be stressed here that the gating time duration must be limitedto a very small fraction of time. I This time interval must be very muchsmaller in practice'than the time constantusually'allotted to squelchcircuits, since the length of time that the mind'of'the hearer canpsychoacoustically fill in an omitted sound without becoming consciousof an interruptionis very small. I I

Fig. 2 shows the circuit of Fig. 1 with similar parts similarly numberedand with'the addition of a low level squelching circhit ll t o f' astandard type forreducing or eliminating'the' fraction of the signalcompletely under the noise level. This will heIp reduce the moredominant noise factor as mother systems. More noise may be tolerated inthis arrangement than-with a standard circuit because the overall noiseis ultimately decreased during the vowel interval and the fragment ofnoise? that is passed also contains a high factor of" intelligibility.

In Fig. 3 the circuit has been varied by the addition'o'f a filtercircuit 220 before the gate actuating squelch cirintroduced bytheelectronic'circuitry in any=of several ways well known in the art.The peaks of energy ofthe signal are detected'by'the peak detectingcircuit which then closes the gating circuit to "reduceor eliminate thepeaks of energy normally applied to the loudspeaker or earphone 16 and,ultimately, the ear. I 1 :3;

Fig. 2 is a similar but is supplemented by the low level squelchingcircuit 124 which maybe of any offthe types well knownjin noisereducing'systems. 4

L Fig. S-isagain similar but has a low-pass filter 220 preceding'thepeak detecting circuit to assure that low frequencyor vowel peaks;.and.-not the consonant peaks actuate the gating circuit. 1 I

. ep h r s t t t co ona t n sxis a ually of prime importance in speech,intelligibility whereas the vowel energy is relatively less important.The voltage peaks of bothvowels: and consonantsmaysbeisubr stantiallythe, same or flle peaks of consonant; voltage maybe even higher than thepcaksoi vowel yoltage, but the consonants appear as sharp ;t ransientsot extremely short time duration, while ,the yowelsounds are ofconsiderably, longer duration. Therefore, themean energy is muchhigherduring a vowel intervalthan during a consonant interval.Theetiectcn the ear where the gain is turned up under difficultconditions rwould be somewhat of a saturation of the; ear from vowelsound to yowel sound to make the recognition of the consonant transientpeaks even lesslikely. This ,rejsults in an overall decrease inintelligibility. In the subject invention the vowel peaks are detectedand madeto actuate a gating circuit so theycan be reduced or eliminatedin thetransdu'certo gvoiddistracting the ear from its ,morefimportanttask of catching the consonants. 1 Theimportance of consonant energyinintelligibility is emphasized by rnany authorities anditis discussed andproven, for example, in the work otLicklider and Miller, beginning onpage;10481oftheffHandbook of Experimental Psychology, edited by. ,S. S.Stevens and published by John Wiley 8: Sons in 1951'. The ,same articlealso points. out that the frequencies of speech under 1000 cyclescontain 80% of the power while contributing only 10% to the articulationof speech.. 1

Relating this to applicants device, if the peak detecting circuit isused without filtering, as inFig. 1, it can be assumed, that 80% oftheactuating. signal energy will be contributed by the lower frequencies;and, in any case, the removal of the portion of the energy sections ofthe speech may only decrease the intelligibility, by;1 On the otherhand, the elimination of the noise for a substantial portion ofthe,reception.time ,wil l ;inerease the intelligibility considerably morethan thedecrease from altering e p ra m. v The well known oorrelationpffrequencies and vowel sounds is discussed in much detail by Fletcher inchapter 5. p ges 282-286 of his volume on1 .'.Spee ch and Hearing,published by Van Nostrandt C ,o Inc. in 1929. Fletcher also showstypical electronic high andlow-pass filters and shows the results offiltering on the'various components of speech. n I i I According to theteachings of thesetexts; and others, most of the energy in speechiscarried in a portion of the frequency spectrum that contributes theleast to the intelligibility. This portion iofthe frequency spectrum isthe lower portion, for the mosttpart below 1000 cycles and, generallyspeaking, comprising vowel sounds.

It is obvious from theteachings of these texts that a substantialportion of .thGYOWOl sounds may be subtracted from the consonant soundsby' either detecting peak; energy or by frequencv discrimination. orboth. On the other hand, it is fairly apparent that since this inventiondeals with psychological acoustical phenomena itisnot limited :to vowelsounds alone butincludestthe efiects of peak energy orlow frequency tofthis described system allowsincrease of the average signalintensitysound level and thereby increases the average power that'm'aybedelivered under conditions reception'close to.the' threshold .of pain.In some ways thisinvention is the aural lequivalentof. .automatic.speedwritingireading. r y

As an additional feature during the cut-off period of the vowel soundsan artiticial substitute tone of' 'approxiinately cycles per second; "beadded to the output to provide. a mental fill-in inerase the length oftime i that the ear will tolerateinterprttttionsi "This'ispds ime inpractice, since the eneaeonsu n enmit a: the ear and mind to identifywen known words make the mind substitute the probably vowel sounds forthe artificial substitute tone.

A further modification of this signal reception technique might beparticularly applicable for extremely 'slow speech where the vowelsounds could be reduced only to the average level rather than completelyeliminated. This would require the gating circuit to be lay-passed byattenuated vowels signals.

It is also conceivable that a second channel of information, notnecessarily speech, may be transmitted during the speech vowel gap timeand thereby accomplish multiplexing.

Having thus described my invention, what is claimed is: I

1. In a system for increasing the intelligibility of speech in thepresence of noise, an amplifier having an input and two outputs, agating circuit having an input, an output and a control connection, apeak detecting circuit for transforming alternating current energy peaksof audio frequencies into direct current switching pulses, saiddetecting circuit having an input for low frequency audio signals and anoutput forthe direct current switching pulses a low pass filterconnecting the input of said detecting circuit to one of the outputs ofsaid amplifier, the output of said detecting circuit connected to thecontrol connection of said gating circuit, the input of said gatingcircuit connected to the other of the outputs of said amplifier, and anelectromechanical transducer connected to the output of said gatingcircuit, said gating circuit being in an on position when no signal isbeing detected and in an off position when a peak energy signal is beingdetected by said detecting means.

2. In a system for increasing the intelligibility of speech in thepresence of noise, an amplifier having an input and two outputs, asource of signals connected to said input, a gating circuit having aninput, a control connection and an output, a low-pass filter for lowfrequency audio signals having an output and an input connected to oneof the outputs of said amplifier, a detecting circuit for transformingalternating current energy peaks of audio frequencies into directcurrent pulses, said detecting circuit responsive to energy peaks abovethe average level, said detecting circuit having an input for audiofrequency signals and an output for direct current switching pulses, theinput of said detecting circuit connected to the output of said low-passfilter, the output of said detecting circuit connected to the controlconnections of said gating circuit, a low level squelch circuit havingan input connected to the other of the outputs of said amplifier and anoutput connected to the input of said gating circuit for cutting offsignals below a given level substantially below said average level, saidgatingcircuit being in an on position when said signals are between thesaid average level and said given level.

3. A squelch circuit for increasing the intelligibility of, speech inthe presence of noise comprising a signal source, an audio frequency lowpass' filter, coupled to said source, a transmission path coupled tosaidlsiourcccontrol means in said path controlling the transmission ofsignals through said path, a gating means to actuate said control meansto block said path, a circuit adapted to be actuated by signalenergyfrom saidlow pass filter exceeding a given levelto produce-arectangular pulse, and an audio frequency low pass filter means forapplying a, portion of said pulse to said gating circuit to actuateitupon the signal energy exceeding said level, said gating circuit actr'ing to block said transmission path when actuated bysaid pulse and tounblock said transmission path at other times when no high energy signalis actuatingsaid circuit.

References Qitedin thefilc of this patent

